A connector assembly has a first connector (10) fit into a receptacle (44A) of a second connector (40). An erroneous connection preventing rib (22) and two fitting ribs (21) are provided on an outer surface of a housing (12) of the first connector (10). The erroneous connection preventing rib (22) can enter a groove (47) on an inner surface of the receptacle (44A) during proper connection of the first and second connectors (10, 40), but collides with the receptacle (44A) during erroneous connection. The fitting ribs (21) fit into fitting grooves (48) on the inner surface of the receptacle (44A) both during the proper connection and during the erroneous connection. The fitting ribs (21) fit into the fitting grooves (48) before the collision of the erroneous connection preventing rib (22) with the receptacle (44A) during the erroneous connection.